Figure that the roots go out at least as far beyond the drip line as to the drip line is from the trunk. Now forget that, because you are NOT gowing to dig up all those roots. If the tree is about 8-12' tall, and has a spread of about half that, then you want to dig your root ball at least 3' across, more if the tree is bigger. Water the tree well a couple of days before you want to move it, so it's well hydrated, and so you can dig in the soil.

If you are in sandy soil, dig a bit wider, and be aware that the sand may not hold together and may just fall off the roots. If this happens, wrap them up anyway as soon as you have dug them free, so they dry out as little as possible, and replant as soon as possible.

Dig it out, severing the larger roots with a saw or an axe if very large, or with loppers if medium large - DO NOT use a chain saw on in-ground roots!! - as you come to them. continue to dig, going round and round the root ball. Dig under it, once it seems you have dug below the most of the sideways roots. Have a tarp, rope and a means of transporting the tree handy, as well as at least one other person, maybe two, as this will be heavy, considering the tree and the rootball dirt. Once the tree is free in the hole, then carefully work the tarp under it - try to lift it by the root ball and not manhandle it by the trunk, as convenient a hand-hold as it is, since that puts a strain on the root ball and the roots. Tie up the tarp, making handles, if you can. Lift it out, again using the root ball, or the rope handles, and transport it to the new hole, which you should have at least started. Measure the root ball, now that you have it next to its new home, anad adjust the dimensions of the hole as needed. Refill the hole - save the excess dirt for another project. Water it well, about half way through, and then water it again when you are done.

Mulch it well, as far out as you are able, but at the least beyond the root ball, no more than 4" of mulch deep, and no mulch in the 2-4" next to the trunk. Plan on watering every few days in the first week or two, and then taper off to once a week, as the new roots begin to grow. A long, slow and deep watering is needed to get the entire root ball wet. Check the top inch or two of soil before you water, if it's dry water, and if it's wet, check again in a day or so. Plan on the equivalent of an inch of rain per week, so adjust the amount you water according to any rainfall. Water for the first year after transplanting. I hope it makes it.